Hitherto, a process for preparing a filler-containing polytetrafluoroethylene (PTFE) granular powder by granulating a mixture of a filler and a PTFE powder with stirring in water was proposed, for example, in JP-B-8611/1968, JP-B-22619/1969, JP-B-37576/1973, JP-B-17855/1974, JP-B-8044/1981, JP-B-18730/1982, etc.
However, by the process for preparation disclosed in each of the above-mentioned patent publications, a filler-containing PTFE granular powder having a small particle size and a narrow particle size distribution could not be obtained.
For that reason, in case of preparing, for example, small articles such as seal rings in the form of a rubber band, thin moldings and molded products having a small surface roughness, there was a problem such that complicated and uneconomical steps must be employed, such as step of sieving a filler-containing PTFE granular powder to take out only particles of small size and molding them, or step of cutting the obtained molding.
Also a granular powder having excellent powder flowability cannot be obtained by only pulverizing a filler-containing PTFE granular powder.
In JP-B-21694/1985, there was proposed a process for preparing a filler-containing PTFE granular powder by granulating, with stirring in water, a PTFE powder and a filler which has been previously surface-treated with an aminosilane compound in coexistence with a water-insoluble organic liquid and an anionic surfactant. However, an apparent density of the filler-containing PTFE granular powder and tensile strength of molded products obtained therefrom are not fully satisfactory.
Further, in the preparation process as mentioned above, there were problems that long steps are employed, a complicated step of pre-mixing the filler and the PTFE powder is required and cost is high.
The present inventors have made intensive studies in view of the above-mentioned problems, and as a result, have found that those problems can be solved by mixing the PTFE powder and the filler in water in the state of slurry in the presence of a surfactant and further granulating the mixture in the presence of an organic liquid which forms liquid-liquid interface with water, with stirring.
Namely, an object of the present invention is to provide a filler-containing PTFE granular powder having a good processability and a process for preparation thereof. Particularly an object of the present invention is to provide a filler-containing PTFE granular powder having a large apparent density, a small average particle size, a narrow particle size distribution and good physical properties such as powder flowability, which gives molded products having excellent physical properties such as elongation, and a process for preparation thereof.